1. Field of the Invention
This invention relates to a throttle system for a general-purpose engine, particularly to a general-purpose engine throttle system that is equipped with a throttle valve connected to an actuator to enable control of the amount of intake air supplied to the engine by operating the actuator to open and close the throttle valve.
2. Description of the Related Art
The general-purpose engine is a spark-ignition internal combustion engine that sucks air into the cylinders, ignites and burns an air-fuel mixture produced by mixing gasoline fuel and an amount of intake air regulated by a throttle valve. The engine speed is generally controlled by a throttle system driven by a mechanical governor comprising weights and a spring.
Still, even in this type of general-purpose engine, highly accurate engine speed control has recently been introduced through the use of an electronic governor connected to the throttle valve through a stepper motor, linear solenoid or other such actuator.
Actuator-driven system throttle systems are taught, for example, by Japanese Laid-open Patent Applications No. Hei 10(1998)-47520 and No. 2001-263098. These prior art systems use a motor as the actuator. The output shaft of the motor and the rotating shaft of the throttle valve are connected through gears so as to transmit the motor rotation to the throttle valve.
As shown in FIG. 11, however, the change in amount of intake air (indicated as xe2x80x9cGairxe2x80x9d) passing through the throttle opening with change in throttle opening increases with decreasing throttle opening (indicated as xe2x80x9cxcex8thxe2x80x9d) and, conversely, decreases with increasing throttle opening. This is because the pressure difference between the upstream and downstream sides of the throttle valve decreases with increasing throttle opening to finally reach a constant minimum value.
In order to follow the desired engine speed with good accuracy and response, therefore, fine throttle valve regulation is required at small throttle opening and high speed throttle valve opening/closing regulation is required at large throttle opening.
Thus, owing to the limited resolution of the motor (stepper motor), the reduction gear ratio must be set high to enable opening and closing of the throttle valve to be performed with finely. On the other hand, the reduction gear ratio must be set low to increase the throttle valve opening/closing speed.
As the reduction gear ratio is constant in the prior art, however, the throttle opening (xcex8th) varies linearly with motor rotation angle (indicated as xe2x80x9cxcex8mxe2x80x9d) as shown in FIG. 12. In the prior art, therefore, an attempt to conduct valve opening/closing finely by setting the reduction gear ratio high runs into the problem that the opening/closing speed becomes so slow as to degrade the engine speed control response when the throttle opening is large.
Conversely, an attempt to increase the throttle valve opening/closing speed by setting the reduction gear ratio low runs into the problem that the fineness of opening/closing at small throttle opening is degraded to the point that accurate control of engine speed becomes impossible. It also encounters the problem that the throttle valve drive torque decreases to the point that throttle valve seizing (sticking) is likely to occur when the throttle opening is small.
An object of the present invention is therefore is to overcome the aforesaid problems by providing a throttle system for a general-purpose engine that can finely open and close a throttle valve when its opening is small (when pressure difference between upstream and downstream of the throttle valve is large), can open and close the throttle valve at high speed when its opening is large (when pressure difference between upstream and downstream of the throttle valve is small), and can prevent throttle valve seizing.
In order to achieve the foregoing object, this invention provides a throttle system for a general-purpose engine, having an actuator connected to a throttle valve of the engine, the actuator being displaceable to open or close the throttle valve so as to regulate amount of intake air, comprising: an output transmission mechanism provided between the actuator and the throttle valve to transmit an output of the actuator to the throttle value such that an output of the mechanism relative to the output of the actuator when the throttle valve is closed is smaller than that when the throttle value is not closed.